The present invention relates to a method for recovery of rhodium from an organic solution containing a rhodium complex having at least one type of organophosphorus compound as a ligand and an organophosphorus compound.
Recently, several methods have been developed wherein soluble complexes of rhodium and organophosphorus compounds are used as catalysts in a homogenous catalytic reaction. These complexes are useful for various reactions such as hydrogenation of olefines carbonyl compounds, and aromatic compounds, etc., hydroformylation and hydrocarboxylation of olefines. Also, since aforesaid complexes are chemically very stable, the catalyst solution can be recycled in the reaction system after separating it from the reaction products by distillation.
However, since various high boiling by-products are formed in the aforesaid reaction and also the catalyst used in the reaction is partially inactivated, a part of the catalyst-containing residue obtained at the recovery of the reaction products by distillation must be removed continuously or intermittently to prevent the accumulation of high boiling by-products and the inactivated catalyst in the reaction medium. The catalyst-containing residue contains an expensive Group VIII noble metals and hence it is necessary to recover the noble metal efficiently.
Generally, to recover the noble metals a wet type recovery method can be used, in which sulfides are added to an organic solution containing noble metals such as rhodium, to form sulfides of the noble metals which are then recovered by precipitation. As an alternative method, noble metal compounds are reduced to metal by addition of a reducing agent. As another alternative, the noble metals are adsorbed to an adsorbent such as an activated carbon.
In still another method, the dry-type recovery method, an organic solution containing noble metals is combusted by using a combustion furnace or in-water combustion furnace to form ash from which rhodium can be recovered.
However, when using the conventional wet-type recovery method, it is not easy for rhodium complex with organophosphorous compounds to react with the sulfides or reducing agent; this is because the organophosphorus compounds are strongly bonded chemically with the noble metal complexes and the rhodium complexes are stable. And, the rhodium compounds are not absorbed sufficiently onto the activated carbon. It thus becomes a practical problem to recover rhodium efficiently and effectively. Moreover, the waste chemicals and exhausted water need to be properly treated as well. And the content of recovered noble metals is relatively low. These are technical drawbacks associated with the conventional wet-type recovery methods.
On the other hand, there are also drawbacks in the dry-type recovery method. The combustion using furnace loses a portion of rhodium, and the obtained ash has a lot of impurities. Even if an in-water combustion furnace which demonstrates relatively high efficiency is employed, a portion of rhodium will be splashed and vaporized during the combustion process, and another portion of the rhodium will be dissolved in the combustion liquid (namely, phosphorous aqueous solution), resulting in a low recovery percentage.